OBJECTIVES of this project are to develop and perfect a method for labelling saxitoxin to high specific radioactivity and to use this toxin (*STX) to study sodium channels in excitable membranes. The specific aspects proposed in the original grant include: 1. labelling, purification and assay of the toxin; 2. altering the toxin structure to determine functional groups & studying exchange rates of 3H from toxin to identify the binding groups on the toxin; 3. studying sodium channel chemistry using *STX and a combination of organic cations and protein reagents; 4. studying the binding of STX to single nerve cells and axons to calculate the conductance of a single sodium channel; 5. a. covalently binding the toxin to sodium channels, and then b. doing autoradiography of single cells ad axons. BIBLIOGRAPHIC REFERENCES: Cohen, Ira S. & Strichartz, Gary R. (1977) On the voltage-dependent action of tetrodotoxin. Biophys J. (in press). Ritchie, J.M., Rogart, R.B. & Strichartz, G.R. (1977). The binding of radioactively labelled saxitoxin to the squid giant axon. submitted to Biophys. J.